Pulsed fiber laser systems can be based on a Master-Oscillator Power-Amplifier (MOPA) architecture where the desired output optical pulses are generated from seed optical pulses emitted from a seed laser. The seed laser, typically emitting pulses of low energy (Master Oscillator) can be, for example, a pulsed semiconductor diode laser, a pulsed low-power fiber laser, a pulsed LED (Light-Emitting Diode), a pulsed solid-state laser, or even a CW laser source, a LED or an ASE (Amplified Spontaneous Emission) source coupled to an amplitude modulator. The seed optical pulses are subsequently amplified in at least one fiber amplifier unit (Power Amplifier), thus increasing the pulse energy while preserving most of the optical characteristics of the original seed pulse.
There are however optical effects that occur in the fiber amplifiers and which can alter or distort the seed pulse. These effects include nonlinear effects that are sensitive to the peak power of the optical pulses such as SPM (Self-Phase Modulation), SBS (Stimulated Brillouin Scattering), SRS (stimulated Raman Scattering), and linear effects such as gain saturation.
In some applications, gain saturation is the optical effect that has the most significant impact on the waveform (temporal profile) of the optical pulses emitted from a pulsed fiber laser. To mitigate these effects, it is known in the art to pre-compensate the waveform of the seed pulse to take into account the pulse distortion brought by this effect (see for instance U.S. Pat. No. 8,073,027 to Deladurantaye et al., “DIGITAL LASER PULSE SHAPING MODULE AND SYSTEM”).
It can be useful to provide a laser system that can emit optical pulses with an arbitrary waveform which is maintained or controlled dynamically during the operation of the system. This type of system would for example be advantageous in laser material processing applications, especially when used in combination with a galvano-scanner system for deflection of the laser beam. A galvano-scanner system uses two mirrors that can be tilted at high speed and, optionally, a dynamically-adjustable beam expander so as to focus an incoming laser beam anywhere over a sample to be processed, through the use of a F-theta lens. The control software of the galvano-scanner computes an optimal path for the laser beam to follow over the sample, depending on the desired end result, and it adjusts the position and speed of the tilting mirrors as well as the current state of the dynamically-adjustable beam expander accordingly. Simultaneously, the control software of the galvano-scanner commands the firing of the laser through the generation of electrical trigger signals for the emission of laser pulses synchronized with the current state of the scanning system.
As a consequence, the repetition rate at which the control software of the scanner triggers the laser can vary dynamically. These variations can even be said to be arbitrary since they are the result of the control software optimization routine for a given process, which is unrelated to the laser operating conditions. This arbitrarily-varying trigger signal is often emitted in the form of bursts, particularly in processes that are non-continuous over the surface of a workpiece.
An example of such a process is the laser inscription of lettering on the surface of a sample. FIG. 2 illustrates an example of an elaborate optical pulse waveform that can be required in applications such as laser material processing and laser marking. The depicted waveform pattern consists in a succession (burst) of 15 sub-pulses having linearly-increasing amplitudes over the duration of the burst, thus resulting in a staircase-like waveform. The sub-pulses have a duration of 10 ns, and they are spaced by a constant time delay of 25 ns. The total energy carried by the burst of 15 sub-pulses is 125 μJ.
To reduce the variability from pulse to pulse in the marks engraved on a workpiece, typical laser systems of current use in the field have features such as “first pulse suppression” or external means for modulating the output beam emitted from the laser as attempts to even out the variations in the energy carried by each laser pulse. There however remains a need for a laser system having the capability to emit laser pulses of controllable energy and waveform in response to an arbitrarily-varying input trigger signal.